This invention relates to improved coatings for composite articles for use in high temperature applications.
Composite articles capable of withstanding exposure to high temperatures have been developed, but there continues to be a need for articles capable of surviving repeated cycles of high temperature and high humidity. It has long been recognized that a major cause of failure of composite articles in high temperature environments is the diffusion of oxygen into the article and the oxidation of the composite matrix, the reinforcing fibers or other elements of the article which are subject to oxidation at high temperatures. There have been many developments aimed at scavenging any oxygen which does penetrate by preferentially oxidizing compounds which form glasses to fill oxygen pathways and prevent further oxygen penetration. Concurrently, composite articles have been coated with inert and impervious layers to prevent the diffusion of oxygen into the composite article. References showing either of both of these techniques include U.S. Pat. No. 4,892,790; U.S. Pat. No. 4,795,677; U.S. Pat. No. 4,894,286; U.S. Pat. No. 4,873,353; and U.S. Pat. No. 4,863,798 as well as commonly assigned application U.S. Ser. No. 07/554,475 now U.S. Pat. No. 5,094,901.
The two primary methods for providing an impervious and inert coating have been application of a polymeric substance which yields a ceramic layer on curing and the direct deposition of a ceramic layer by chemical vapor deposition (CVD). U.S. Pat. No. 4,873,353 is a reference relating to the former method, and U.S. Pat. No. 4,892,790 is a reference relating to the latter. In fact, the '790 patent teaches first an application of ceramic particles in a char-yielding binder and subsequent application of a ceramic layer by CVD. Use of a polymer which converts to a ceramic layer on curing has an advantage of ease of application, but it is not always easy to get a totally impervious coat. Application of a ceramic coat by CVD generally provides a more diffusion resistant layer, but achieving a uniform deposit over the entire surface of an article with complex geometry can be difficult.
Both of the methods described above exhibit some failures when used with composite articles exposed to repeated cycles of high and low temperatures together with exposure to high humidity. It has proven very difficult to completely seal the composite material against invasion of oxygen. Further, since the coatings have different coefficients of thermal expansion than the underlying composite matrix material, microcracks are formed in the outer layer when the article is exposed to repeated cycles of high and low temperature. These microcracks provide additional passages for diffusion of oxygen. Finally, in extremely severe conditions, the articles are exposed not only to temperature extremes, but also to high humidity at low temperatures. Any moisture which diffuses into the layer and condenses at low temperatures is rapidly converted to steam at high temperatures. If the steam is formed faster than it can diffuse out of the article, it can cause the protective layer to spall off the composite article, increasing the avenues for invasion by oxygen.
Thus, there remains a need for a means of protecting composite materials which are to be subjected to repeated exposure both high temperatures and high humidities. Articles which can withstand such conditions are useful in many applications, one example being:in high-performance jet engines. The process of this application provides the required protection for composite articles, and has the advantage of relative processing ease, and excellent dimensional control.